1. Field of the Invention
The present invention relates to a method for control of exhaust gas purification system in an exhaust gas purification system provided with a catalyst device supporting a catalyst for purifying an exhaust gas in an internal combustion engine and an exhaust gas purification system.
2. Description of the Related Art
There are various studies and proposals made on an exhaust gas purification device for purifying an exhaust gas in an internal combustion engine such as diesel engines and a part of gasoline engines have been made. Among them, there is an exhaust gas purification device in which a DPF (diesel particulate filter) or a NOx purification catalyst for purifying NOx (nitrogen oxides) is arranged. As the NOx purification catalyst, a three-way catalyst, an NOx occlusion/reduction catalyst, an SCR catalyst (selective contact catalyst) with urea addition, NOx direct reduction catalyst and the like are used. Also, arrangement of oxidation catalysts on the upstream side of the DPF or NOx purification catalyst is often employed.
In the oxidation catalyst, NOx purification catalyst and the like, a precious metal catalyst is used. With the oxidation catalyst, HC (hydrocarbon), CO (carbon monoxide) in the exhaust gas is oxidized by catalytic action of the precious metal catalyst.
With the NOx purification catalyst, using the catalytic action of the precious metal catalyst, in a rich air/fuel ratio with more fuel (thick air/fuel ratio) state, NOx is reduced to HC, CO, and three exhaust gas components of NOx, HC, CO are purified at the same time. On the other hand, in a lean air/fuel ratio (thin air/fuel ratio) with less fuel state, NO (nitrogen monoxide) is oxidized to NO2 (nitrogen dioxide) and this NO2 is occluded by an NOx occlusion material.
In an exhaust gas purification device in which an oxidation catalyst is arranged on the upstream side of an SCR catalyst, the oxidation catalyst promotes reaction from NO to NO2 and reaction with NH3 (ammonia) on the SCR catalyst.
The precious metal catalyst has the above important role. However, at a high temperature, a precious metal molecule is moved and its grain size is increased, which results in reduced total surface area and an area to which the exhaust gas component is adsorbed. Thus, such a phenomenon occurs that a low-temperature activity is particularly lowered. This phenomenon is called as sintering. If the sintering occurs in the oxidation catalyst, a temperature at which purification by the catalyst starts, in other words, a temperature at which HC, CO purification rates start to function (catalyst light-off temperature) is raised, while a catalyst light-off performance is deteriorated. Thus, the catalyst activity at a low temperature is worsened, the purification rate of the exhaust gas is lowered, and a state of the exhaust gas emitted to the atmosphere is deteriorated.
In the case of the NOx purification catalyst, when the sintering occurs, at a low temperature and in a rich air/fuel ratio state, a three-way purification rate of the three-way catalyst and the NOx occlusion/reduction catalyst at a low temperature is lowered, while in the lean air/fuel ratio state, reaction from NO to NO2 is not promoted but NOx activity is lowered, and the occlusion capacity of the NOx occlusion/reduction catalyst is deteriorated. Thus, the NOx purification rate is lowered.
In the case of combination of the oxidation catalyst on the upstream side and the exhaust gas purification device such as the DPF, if the sintering occurs, oxidation of HC, CO, NO in the oxidation catalyst is not promoted at a low temperature and the temperature of the exhaust gas flowing into the exhaust gas purification device through the oxidation catalyst is not raised any more. As a result, if the exhaust gas purification device is a DPF, PM is easily collected, while if the exhaust gas purification device is a NOx occlusion/reduction catalyst, the temperature of the flow-in exhaust gas can not be fully raised by regenerative control and desulphurization control.
Moreover, in the case of combination of the oxidation catalyst on the upstream side and the SCR catalyst, when the sintering occurs, activity from NO to NO2 in the oxidation catalyst is lowered at a low temperature, which deteriorates the NOx purification rate.
In the PM regenerative control for burning and removing PM collected by the DPF, NOx regenerative control for restoring NOx occlusion capacity of the NOx occlusion/reduction catalyst, desulphurization control for restoring sulfur poisoning of the NOx purification catalyst or the like, temperature raising of the exhaust gas by supplying HC, CO into the exhaust gas by post injection or the like and by oxidizing this by an oxidization catalyst arranged on the upstream side is widely carried out. At this temperature raising of the exhaust gas, the oxidation capacity of the HC, CO at a low temperature in the oxidation catalyst on the upstream side is also lowered. Therefore, a flow-out amount of the supplied HC, CO to the downstream side of the exhaust gas purification device might be increased, or PM regeneration, NOx regeneration, desulphurization or the like might be insufficient.
On the other hand, if the temperature raising control of the exhaust gas is executed by setting a value of a temperature for determination used in the temperature raising control high in advance, taking into consideration of the sintering, costs might be deteriorated by temperature rise more than necessary, or the sintering might be accelerated by the exhaust gas whose temperature is raised more than necessary, which results in shorter life of the catalyst.
Therefore, it is important to detect or determine a deterioration state of the oxidation catalyst or the like. As a deterioration detecting method of the exhaust gas purification catalyst, a method of detecting a deterioration state of the exhaust gas purification catalyst as described in Japanese Patent Kokai No. H09-164320 is proposed by detecting a rise of the activation temperature of the catalyst, detecting a rise of the temperature of the exhaust gas required for activation of the catalyst, or by detecting if a difference between the temperature of the exhaust gas flowing into the catalyst and the temperature of the exhaust gas flowing out of the catalyst (or catalyst temperature) has reached a predetermined value or not.
As another deterioration degree determining method of NOx catalyst, an exhaust purification device for an internal combustion engine as described in Japanese Patent Kokai No. H10-259714 is proposed that in the NOx catalyst, while in a NOx purification temperature window, the deterioration of the NOx catalyst is determined from a change of the NOx purification rate at a time when HC supply concentration is increased in a short time during which the catalyst temperature is not changed.
This NOx purification rate is calculated by η1=(S1·S2)/S1, η2=(S1·S3)/S1 from an NOx concentration S1 at a catalyst-inlet gas calculated based on an output signal of an accelerator sensor and an engine speed sensor and an NOx concentration S2 (or S3) at a catalyst outlet detected by an output signal of an NOx sensor. A change amount of the NOx purification rate is calculated by (η2−η1), which is compared with a deterioration determination value set in advance.
In any of the deterioration determinations, only determination on deterioration or not is made by comparing numeral values for determining deterioration such as temperature difference and change in the NOx purification rate with the deterioration determination value set in advance. The deterioration determination is an effective deterioration determination method since it plays an important role in determining timing of desulphurization control for restoring from sulfur poisoning of the catalyst or timing for replacement of the catalyst.
However, since majority of the deterioration of the catalyst is caused by thermal deterioration such as sintering and deterioration by sulfur poisoning, it develops gradually in many cases, and the PM regenerative control, NOx regenerative control, desulphurization control or the like in conformity with the gradually developing deterioration is particularly important from the viewpoint of fuel cost improvement. Thus, detection or determination of a degree by which the activation characteristics (light-off temperature) is shifted to a high-temperature side (deterioration degree) due to development of the catalyst deterioration is more important than whether or not the catalyst has been deteriorated or not. However, there is no proposal made on this point in the above deterioration determinations.
Moreover, with the former deterioration detecting method (Patent Document 1), the catalyst deterioration is checked using a temperature at which the catalyst activation is started as determination standards, but this temperature at which the catalyst activation is started is largely changed by catalyst spatial speed, supplied HC concentration, and catalyst temperature rise per unit, and there is a problem of poor deterioration determination accuracy. On the other hand, with the latter deterioration determination method (Patent Document 2), the NOx sensor has not been put into practical use yet for diesel engines at present, its reliability is low and costs are high, which are problems.
Patent Document 1: Japanese Patent Kokai No. H09-164320
Patent Document 2: Japanese Patent Kokai No. H10-259714 (page 6, the paragraph [0040] to page 7, the paragraph [0045])